ipv6: tcp: consistently use MAX_TCP_HEADER

All other skbs allocated for TCP tx are using MAX_TCP_HEADER already.

MAX_HEADER can be too small for some cases (like eBPF based encapsulation),
so this can avoid extra pskb_expand_head() in lower stacks.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reviewed-by: NDavid Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20220222031115.4005060-1-eric.dumazet@gmail.comSigned-off-by: NJakub Kicinski <kuba@kernel.org>

net/ipv6/tcp_ipv6.c

@@ -925,12 +925,11 @@ static void tcp_v6_send_response(const struct sock *sk, struct sk_buff *skb, u32
 	}
 #endif
 
-	buff = alloc_skb(MAX_HEADER + sizeof(struct ipv6hdr) + tot_len,
-			 GFP_ATOMIC);
+	buff = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC);
 	if (!buff)
 		return;
 
-	skb_reserve(buff, MAX_HEADER + sizeof(struct ipv6hdr) + tot_len);
+	skb_reserve(buff, MAX_TCP_HEADER);
 
 	t1 = skb_push(buff, tot_len);
 	skb_reset_transport_header(buff);